rustc: Correctly pretty-print macro delimiters

This commit updates the `Mac_` AST structure to keep track of the delimiters
that it originally had for its invocation. This allows us to faithfully
pretty-print macro invocations not using parentheses (e.g. `vec![...]`). This in
turn helps procedural macros due to #43081.

Closes #50840

src/libsyntax/ast.rs

@@ -1239,9 +1239,17 @@ pub type Mac = Spanned<Mac_>;
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct Mac_ {
     pub path: Path,
+    pub delim: MacDelimiter,
     pub tts: ThinTokenStream,
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub enum MacDelimiter {
+    Parenthesis,
+    Bracket,
+    Brace,
+}
+
 impl Mac_ {
     pub fn stream(&self) -> TokenStream {
         self.tts.clone().into()